Evacuating system for mass spectrometry



NOV. 25, 1947. w, w s I 2,431,355

EVACUATING' SYSTEM FOR MASS SPECTROMETERY Filed July 5, 1943 2 Sheets-Sheet 1 1/ 6? l I I I 1 v 7 i. AMPLIFIER V 6? 2/ Hero/e052 ATTORNEY-5 Patented Nov. 25, 1947' OFFICE EVACUATING SYSTEM FOR MASS SPECTROMETRY Harold W. Washburn, Pasadena, Calif., assignor Consolidated Engineering Corporation,

Pasadena, Calii'., a corporation of California Application July 3, 1943, Serial No. 493,361

7 Claims. 1

This invention is concerned with mass spectrometers and particularly with a system employed for evacuating the analyzer chamber and other parts of a mass spectrometer which are operated at low" pressures and the surfaces of which should be protected against contamination.

A mass spectrometer is an apparatus employed for sorting ions. Ordinarily it includes an ionization chamber in which molecules of amaterial to be analyzed are bombarded by a stream of electrons and converted into ions. pelled into and through an analyzer chamber, where they are acted upon by a magnetic or an electric field or both and separated according to their mass-to-charge ratios, 1. e. their specific masses, into a plurality of diverging beams of ions having different specific masses, with each beam composed of ions of the same specific mass. The diverging beams are successively collected and discharged at an ion collector and the current thus produced from each ion beam is indicative of the amount of ions in that beam, and may be a measure of the proportion of the molecules (from which the ions were derived) present in material undergoing analysis.

The ionization chamber and the analyzer chamber should be operated at a low pressure, of the order of 0.1 and .001 micron, respectively, so that the mean free path of each and every kind of ions is greater than the distance that the ions have to travel from their point of origin to the point of their collection.

Conveniently, the required low pressures are obtained by enclosing the ionization chamber and the analyzer chamber in a sealed envelope and evacuating the envelopethrough a conduit connected to a vacuum pump. Various types of pumps, such as diffusion pumps or molecular pumps that are capable of accomplishing a high degree of evacuation, may be employed.

In my work in mass spectrometry I have found that it is extremely important to the reproduci- 'bility of the spectra obtained and the accuracy of mixture analysis that asuillcient degree of vacuum be obtained and that the interior surfaces of the spectrometer, especially the surfaces of the analyzer chamber and the ionization chamber, particularly the latter. be kept as free as possible from contamination. I have also found that when a diffusion pump is employed,

These are prothe there is considerable danger of contaminating the interior of the apparatus by the oil, mercury, or the like which is employed as a pumping medium and have developed a simple apparatus for avoiding this contamination.

In accordance with my invention I interpose a series of refrigerated traps in the conduit connecting the mass spectrometer to the diffusion pump (i. e. the conduit which communicates with the analyzer chamber and serves to evacuate it) with a valve disposed in the conduit between the ionization and analyzer chambersand the traps. Means are provided for keeping the traps progressively cooler in the direction of the spectrometer. Thus, if a series of two traps are employed, that nearest the ionization chamber and analyzer may be cooled with a refrigerant, such as liquid nitrogen, capable of making that trap cooler than the other trap (i. e. that one near the diffusion pump) which is cooled by"dry ice," or other refrigerant which is inherently incapable of refrigeratin as low as the liquid nitrogen. Preferably an additional vacuum pump is connected in the system beyond the difiusion pump for handling the exhaust thereof.

By diffusion pump I mean a pump of the type illustrated in Fig. 1B and comprising an inlet conduit for the vapor to be exhausted, an aspirating chamber connected to the conduit, 3, reserv'oir for liquid connected tothe aspirating' chamber, means for heating the liquid to vaporize it and force it past the inlet conduit in the aspirating chamber to force the vapor from the inlet conduit into the aspirating chamber and suck'it from the system. Preferably such a diffusion pump has a condenser connected to its outlet for condensing the liquid, with means for returning the condensed liquid to the reservoir.

The foregoing and other features of my invention will be understood more thoroughly in the light of the following detailed description, taken in conjunction with the accompanying drawings, in which: a

Fig. 1A illustrates schematically a mass spectrometer adapted to be equipped with a pumping system constructed in accordance with my invention;

Fig. 1B is a diagram showing a pumping system of my invention adapted for connection to the spectrometer of Fig. 1A; and I Ivelope.

Fig. 2 illustrates in detail an internally operative valve employed in the apparatus of Fig. 13.

Referring now to Fig. 1A, it will be observed that it shows a mass spectrometer comprising an ionization chamber ll, an analyzer tube l2, and an ion collector I3 disposed within an envelope I which must be kept at a low pressure during operation. The spectrometer may be of any appropriate design but conveniently is constructed as disclosed in my co-pending application Serial No. 513,527, filed December 9, 1943.

The apparatus of Fig. 1A is provided with a pumping system or envelope exhaust line l5 which is connected to the envelope by a tight spherical joint I5A. As shown in Fig. 1B, the exhaust line includes an externally cooled trap I6 adapted to be cooled by evaporation of liquid nitrogen in an external jacket ISA, an internally cooled trap l1 adapted to be cooled by evaporation of dry ice in a well "A, a first down draft mercury diffusion pump l8, a second down draft mercury diffusion pump l9 and a high vacuum pump 20, all connected in series in the order named. An internally operative valve 2| of novel construction is disposed in the exhaust line between the envelope and the liquid nitrogen trap.

If desired, the apparatus may be provided with an ionization gauge 22 communicating with the conduit between the envelope and the valve. A Pirani bulb 23 may be connected with the conduit between the high vacuum pump and the second diffusion pump, if desired,

The diffusion pumps are of conventional design-and comprise a boiler A adapted to contain mercury (or oil) and to be heated by a filament B, Vaporized mercury passes from the heater through a conduit C into an aspirator D. The mercury vapor issuing from the aspirator carries with it gas to be exhausted from the system. Thegas mixed with mercury vapor passes into a jacketed water cooled condenser E from whence the mercury returns to the boiler while the gas is withdrawn through a conduit F.

All gases having low vapor pressures, i. e. vapor pressures below about mm. Hg at the temperature of liquid nitrogen condense on the refrigerated wall of the liquid nitrogen trap, so that the total vapor pressure in the conduit is reduced at this point. Gases wliieh are noncondensable at the temperature of liquid nitrogen are evacuated through the mercury diffusion pumps and the Hyvac pump.

Inevitably, some of the mercury vapor tends to diffuse upstream, i. e. back toward the en- But this mercury vapor is prevented from reaching the envelope and thereby reaching the analyzer and the ionization chamber by means of the refrigerated traps. Some mercury vapor does diffuse upstream beyond the dry ice trap, however, and accumulates within the liquid nitrogen trap, but does not pass further.

In normal operation, the mass spectrometer is kept continuously at low pressure by continuously operating the pumping apparatus.

In the operation of the spectrometer gases, for

, example, hydrocarbons are exhausted from the envelope of the spectrometer and condensed and accumulated 0n the refrigerated wall of the liquid nitrogen trap. After a substantial quantity uid nitrogen, or dry ice both traps and exhausting the vaporized hydrocarbons through the line and permitting the mercury that is released from the dry ice trap to return to the boiler of the first mercury diffusion P p.

A small quantity of mercury will remain within the liquid nitrogen trap but its quantity is so small that even the accumulation of several months is not sufllcient to require cleaning.

During the trap cleaning process just described, it is necessary to close the valve between the liquid nitrogen trap and the envelope to prevent the vaporized hydrocarbons and other gases as well as mercury from entering the envelope. The construction of this valve will be more easily understood through consideration of Fig. 2. As shown in this figure, the totally enclosed internally'operated valve 2| comprises a main body section 30 having a tubular section 3| that forms part of the conduit with a restricted throat 32 on the downstream side. The face of the throat on the downstream side is ground smooth and forms a flat valve seat 33 against which a valve plate 34 may beheld to close the envelope system from the pumping system and thereby protect the envelope and the apparatus which it contains.' In this way, contamination of the analyzer surfaces and the surmoved axially by means of a magnet (not shown) or the like applied externally of the valve. Thus, external magnetic forces may be employed to move the valve plate along the axis of the stem away from the seat or toward the seat. Hooks 4|, 42 secured to the armature are provided for engaging one or the other bracket, thereby holding the valve plate away from the valve seat or tightly against it as the case may be.

Because of the necessity of maintaining a high vacuum in the system, the totally enclosed internally operative valve is highly desirable, since it is completely sealed against the exterior and offers no opportunity for gas leakage into the system.

I claim:

1. A system forevacuating a chamber comprising an evacuation conduit communicating with the chamber, a diifusion pump connected with the conduit for evacuating the chamber, a series of vapor traps disposed in the conduit between the chamber and the pump, means for externally coolin the traps and to cool the trap nearest the chamber to a temperature lower than that of the other traps, and a, valve disposed in the conduit between the nearest trap and the chamber, said valve having a seat and valve stem entirely within the conduit and having a magnetic member on the stem for operation by externally applied magnetic means.

2. A system for evacuating a chamber comprising an evacuation conduit communicating with the chamber, a diffusion pump connected with the conduit for evacuating the chamber, a series of vapor traps disposed in the conduit between the chamber and the pump, means for externally cooling the traps and to cool the trap nearest the as the case may be) from Q chamber to a temperature lower than that of the other traps, and a valve disposed in the conduit between the nearest trap and the chamber, said valve having a seat and a valve stem entirely within the conduit, a member of a magnetic material on the stem for operation by externally applied magnetic means, nd a spring member between the stem and the valve.

3. An evacuating system for a mass spectrometer comprising an evacuating conduit adapted to communicate with the chamber of the mass spectrometer to be evacuated, a diffusion pump connected with the conduit for evacuating the chamber, a plurality of vapor traps disposed in series in the conduit between the chamber and the diffusion pump, means for refrigerating the walls of the traps and for making them progressively cooler in the direction of the chamber and a valve disposed in the conduit between the chamber and the first of said traps, said valve having a seat and a valve stem positioned entirely within the conduit and being adapted to operate by externally applied magnetic means.

4. An evacuating system for a mass spectrometer comprising an evacuation conduit adapted to communicate with the chamber of the mass spectrometer to be evacuated, a diffusion pump connected with the conduit for evacuating the chamber, a series of vapor traps disposed in the conduit between the chamber and the diffusion pump, means for refrigerating the walls of said traps and for making them progressively cooler in the direction of the chamber, and a valve completely surrounded by a conduit wall disposed in the conduit between the nearest trap and the chamber, said valve having a seat and valve stem entirely within the conduit and having a magnetic member forming a part of said stem for operation by externally applied magnetic means.

5. An evacuating system for a mass spectrometer comprising an evacuation conduit adapted to communicate with the chamber of the mass spectrometer to be evacuated, a vacuum pump connected with the conduit for evacuating the chamber, a diffusion pump disposed in the conduit between the vacuum pump and the chamber, a series of vapor traps disposed in the conduit between -the chamber and the diffusion pump, means for refrigerating the walls of said traps so arranged that the trap nearest the chamber is made cooler than those further away, and a valve disposed in the conduit between the nearest trap and the chamber, said valve having a seat and valve stem within the conduit and having a magnetic member forming a part of said stem for operation by externally applied magnetic means.

6. An evacuating system for a mass spectrometer comprising an evacuation conduit adapted to communicate with the chamber of the mass spectrometer to be evacuated, a diffusion pump connected with the conduit for evacuatin the chamber, a first vapor trap disposed in the conduit between the chamber and the diffusion pump, means for refrigerating the walls of said trap with liquid nitrogen, a second vapor trap disposed in the conduit between said first trap and said pump, means for refrigerating the walls of said second trap with a refrigerant having a higher vaporizing temperature than liquid nitrogen, and a valve having all its moving parts completely enclosed by a conduit wall disposed in the conduit between said first trap and the chamber and adapted to be operated by externally applied magnetic means.

7. An evacuating system for a mass spectrometer comprising an evacuation conduit adapted to communicate with the chamber of the mass spectrometer to be evacuated, a vacuum pump connected with the conduit for evacuating the chamber, a difiusion pump disposed in the conduit between the vacuum pump and the chamber, a first vapor trap disposed in the conduit between the chamber and said diffusion pump, means for refrigerating the walls of said first vapor trap with liquid nitrogen, a second vapor trap disposed in the conduit between said first trap and the difiusion pump, means for refrigerating the walls of said second trap with a refrigerant having a higher vaporizing temperature than said liquid nitrogen, a valve disposed in the conduit between said first trap and the chamber, said valve having a seat and valve stem entirely within the conduit, a member of magnetic material on the stem for operation by externally applied magnetic means, and a spring member between the stem and the valve.

HAROLD W. WASI-IBURN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,307,999 Buckley June 24, 1919 2,331,190 Hipple Oct. 5, 1943 1,775,999 Haselofi Sept. 16, 1930 283,544 Weston Aug. 21, 1883 1,876,044 Davis Sept. 6, 1932 FOREIGN PATENTS Number Country Date 498,812 Germany 1930 OTHER REFERENCES Review of Scientific Instruments, volume 11, July 1940, page 214, lines 28-32. 

